It is known that a ramjet, like any other aerobic engine, includes at least one combustion chamber supplied (1) with fuel from a fuel source and (2) an oxidizing agent from air, derived from the ambient atmosphere through at least one ram air inlet, the combustion products being ejected through at least one exhaust nozzle.
For a ramjet to effectively operate, especially as concerns the compression of the combustion air, the aircraft on which it is installed must be moving at a relatively high speed, such as speed being obtained notably by equipping said aircraft with an acceleration propellant grain incorporated or not into the ramjet combustion chamber or by launching from a carrier airplane or a carrier rocket which, themselves, reach the required speed.
Furthermore, it is known to be advantageous for assisting the combustion development and efficiency within a ramjet combustion chamber to distribute as much as possible the fuel in said chamber, this concern having led to the design of a range of ramjets using a liquid fuel, such as kerosene for instance, which is then injected in sprayed form into the ramjet chamber.
The ramjets pertaining to this range, called "liquid fuel ramjet", are undeniably advantageous from the performance standpoint but due to the use of a liquid fuel phase they show some operational difficulties because of the leaks which may occur. Besides, the liquid phase must be pressure-injected thereby increasing the system complexity while reducing its reliability.
In order to cope with these difficulties, it has been proposed to implement another ramjet family, the fuel phase of which is restricted to a solid phase.
This technique can be applied, for instance, by placing the solid fuel phase into the ramjet combustion chamber.
Using a special generator, it is also possible to convert this solid fuel phase into a gaseous fuel phase which is then gradually delivered in said combustion chamber. In the latter case, it is necessary to provide the ramjet with a combustible gas generator using an autopyrolyzable solid composition for the production of such gases.
Solid propellant compositions consisting essentially of a high amount of an oxidizing agent and a weakly oxygenated binder are already known.
U.S. Pat. No. 3,087,844 and 3,883,375, for instance, describe composite solid propellant compositions consisting of a polyalkadiene having functional terminal groups hardened by a triaziridine phosphine oxide and oxidizing agent such as ammonium perchlorate.
These compositions however are difficult to use in aerobic propulsion because they are finely balanced in oxygen and release therefore few reducing gases thus leading to poor performance. In addition, due to the presence of ammonium perchlorate, these compositions are liable to emit secondary smokes which impair the discretion.
According to U.S. Pat. No. 3,756,874, propellants are also known, the oxidizing agent of which is a mixture of potassium perchlorate and cyclotetramethylenetetranitramine (HMX).
But once again, such compositions have too rich an oxidizing agent content to be efficiently used in aerobic used propulsion. Furthermore, the binders used, such as epoxy or polyester resins, are too highly oxygenated to obtain the release of sufficient amounts of reducing gases.
In the specific field of aerobic propulsion, it has been proposed as described in U.S. Pat. No. 3,952,505, to make use of a composition containing a mixture of hydroxytelechelic polyalkadiene and one or several aromatic densifying compounds such as pyrenes or anthracenes. This type of composition however is absolutely not autopyrolyzable. In fact, the combustion can only be achieved with the assistance of oxygen from the air.
Thus, for the present application, it is necessary that the solid fuel phase be in the engine combustion chamber in order to provide the combustion.
Besides, there has been proposed in U.S. Pat. No. 3,754,511 a composition consisting of a mixture of reducing compound, such as boron or aluminum and an oxidizing agent such as niter. But this type of composition shows some inconveniences due to implementation problems. In addition, this solution, although tempting from the theoretical standpoint, requires specific configurations and cannot be used for all missions. Also, a discretion of such compositions is less satisfactory thus restricting their operational capabilities.